2011年英语四级阅读备考材料（13）_第2页

Condensation. Fortunately, the process of evaporation can reverse itself. Otherwise, the humidity would always be high and precipitation would never take place. The process which is the reverse of evaporation is called condensation and it occurs when the temperature falls. As it becomes cooler, water molecules begin to move more slowly. Collisions take place and molecules hang together to form tiny droplets which are no longer invisible.

　　Cloud Formation. The condensation process explains how clouds are formed. Warm air, laden with moisture, rises into the atmosphere. As the air rises it tends to expand and this causes the temperature to drop. The falling temperature slows down the speed of molecular movement, and the water molecules gather together on dust particles floating around in the atmosphere. This condensation results in the formation of billions of tiny water droplets that make up a cloud. The droplets are so small and lightweight that they are hardly affected by gravity. Slight air movements keep them floating miles above the earth.

　　Sometimes a pass of moist air close to the earth's surface may cool off rather suddenly. This may happen when a body of cold air moves in or when the moist air drifts over a cooler body of water. When this occurs, condensation takes place, but the cloud which forms is right on the surface of the earth rather than high in the atmosphere. Such a low-hanging cloud is called a fog. In composition the fog is exactly the same as a cloud. The only difference between the two is the place in which each occurs. The cloud is high, the fog forms on the ground.

　　In certain areas of the world, such as Newfoundland and the city of London, fogs occurs frequently. This is because local conditions being about sudden cooling of moisture-laden masses of air rather often.

　　Dew and Frost. As the sun warms the earth during the day, evaporation continues to take place. At night, however, the earth cools down as it gives off some of the heat it has received from the sun.

　　Grass loses its heat especially fast. The air which surrounds the grass is full of water vapor. As the moving water molecules strike the cool blades, they slow down and condensation takes place. Thus, millions of molecules come together on the grass to form the tiny water droplets we call dew. Can you understand why dew forms readily on cool, clear nights? The next time you have an ice cream soda you can watch the process of dew formation. As the cold ice cream lowers the temperature to the glass, water vapor in the surrounding atmosphere condenses on the surface and the glass seems to "sweat". This is artificial dew, perhaps, but is from exactly the same reason that dew forms in nature.